One class of opto-electrical devices is that using an organic material for light emission. The basic structure of these devices is a light emissive organic layer, for instance a film of a poly (p-phenylenevinylene) (“PPV”) or polyfluorene, sandwiched between a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes) into the organic layer. The electrons and holes combine in the organic layer generating photons. In WO90/13148 the organic light-emissive material is a conjugated polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). In a practical device one of the electrodes is transparent, to allow the photons to escape the device. These organic light-emissive devices (“OLEDs”) have great potential for display and lighting applications.
With reference to FIG. 1, a typical OLED is fabricated on a glass or plastic substrate 1 coated with a transparent anode 2 such as indium-tin-oxide (“ITO”) and covered with, in sequence, a hole transporting layer 3, a layer of organic electroluminescent material 4 and a cathode 5. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminium, or a plurality of layers such as calcium and aluminium. Further layers may be provided between the electroluminescent layer and the electrodes in order to enhance charge transport to the electroluminescent layer. This includes electron blocking layers between the anode and the electroluminescent layer, and electron transporting and/or hole blocking layers between the cathode and the electroluminescent layer.
FIG. 2 illustrates the energy levels of the device of FIG. 1. The electroluminescent layer illustrated here comprises hole transport “HT”, electron transport “ET” and electroluminescent “EL” materials. These materials may be provided as a blend within the electroluminescent layer or as components of the same molecule as described in, for example, WO 99/48160.
As shown in FIG. 3 holes are injected into the device through the anode and electrons are injected into the device through the cathode when the device is operated. The holes “float” to the highest occupied molecular orbital (HOMO) of the electroluminescent material and electrons “sink” to the lowest unoccupied molecular orbital (LUMO) of the electroluminescent material and then combine to form an exciton which undergoes radiative decay to give light.
In the above-described device, emission is from one electroluminescent species only. However, devices are known wherein electroluminescence originates from more than one electroluminescent species. In one such device disclosed in WO 99/48160, more than one electroluminescent material is provided in the electroluminescent layer. WO 2006/067508 and U.S. Pat. No. 5,807,627 disclose another arrangement wherein the device comprises more than one electroluminescent layer.
It is obviously desirable to maximize the efficiency of an OLED. This can be done by selection of electroluminescent materials that are inherently highly efficient (as measured, for example, by their photoluminescent efficiency) or optimization of device architecture in order to achieve efficient and balanced transport of holes and electrons.
However, the present inventors have found that use of certain materials in the electroluminescent layer of an OLED comprising multiple emissive species results in unexpectedly low device efficiency and higher drive voltage.